There will be another test

Discussion of my cable tester on https://lookmumnocomputer.discourse.group/ prompted some discussion of an Arduino-based current measuring tool. Knowing how much current your modules use is a good thing, because there’s a limit to how much current a power supply can give you. For commercial modules the designer usually tells you the current requirements, but not always (and not always accurately), and of course for scratch builds you’re on your own. So, yeah, good to measure current.

But I figured that while trying to stick a multimeter on both ends of various conductors in a ribbon cable to check them all for continuity and shorts is an awkward and complex enough process to warrant automating it, just measuring two or three currents by hand is arguably no harder than finding the USB cable you need to plug an Arduino in, and doesn’t involve building a relatively complicated op amp circuit to get two or three voltage differences scaled, rectified, and into a microcontroller’s input pins.

So I’m not much interested in designing something fancy, but the discussion did induce me to actually make something for current measurement, even if not Arduino-based.

Electronic tools don’t come much simpler than this. There are two pairs of box headers — a 16-pin and a 10-pin — connected to each other on a stripboard, with 10Ω resistors in the middle on the +5, +12, and -12 volt rails. There are two headers, 6 pins long but with three of the pins removed, providing contacts for measurement. That’s all. (Of course I did want to make sure the headers were connected up properly before using it. So what did I do? Right. Put cables on both ends and put the other ends of the cables into the cable tester. No shorts, no bad lines, good to go!)

You connect a cable to the power supply on one side and a cable to the module under test on the other. Turn on the power and use your multimeter to measure the voltage drop across each of the resistors in millivolts. By Ohm’s Law, divide that by 10 to get the current in milliamps.

The resistance isn’t exactly 10Ω, my multimeter said about 10.3Ω for each despite their being 1% tolerance — which is probably a multimeter calibration issue. Moreover, since you’re stealing 10 millivolts per milliamp to do this test, the module’s getting slightly less voltage than normal, which could slightly affect its behavior and its current draw. Who cares? If this tells you the current requirements within 5 mA or so, that’s good enough for most purposes.

One thought on “There will be another test

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